1. Field of the Invention
The present invention relates to a motor control system that includes a model control system and performs model following control.
2. Description of the Related Art
Model following control is known as a technique for high-speed positioning of a machine such as a chip mounter using a motor control system. In the model following control, a model control system simulating an actual feedback control system is built up. The feedback control system is driven to follow the model control system. FIG. 2 shows the configuration of a motor control system adopting conventional model following control disclosed in Japanese Patent Application Publication No. 62-217304 (JP62-217304A). In the control system, the deviation between a position command and a model position is acquired and supplied to a model position controller to obtain a model speed command. A deviation between the model speed command and a model speed is then acquired and supplied to a model speed controller to obtain a model torque command. The model torque command is supplied to a motor mechanical model to calculate the model speed. The model speed is supplied to an integrator to calculate the model position. A difference between the model position and a motor position detected by an encoder is acquired and supplied to a position controller to output a speed command. A deviation between a value obtained by adding the speed command and the model speed and a detected speed value is acquired and supplied to a speed controller to obtain a torque command. A value obtained by adding the torque command and the model torque command is supplied to a torque controller to drive a motor, controlling a torque of the motor. The motor mechanical model is represented as 1/{(JM+JL)S} wherein JM represents a motor inertia and JL represents a load inertia. According to the thus configured model following control, command response characteristics and disturbance suppression characteristics can be independently controlled. The disturbance suppression characteristics are restricted by high-frequency resonance of the mechanical system etc., and therefore cannot be improved more than a certain level. Meanwhile, the model response characteristics are not restricted by high-frequency resonance of the mechanical system etc., and therefore can be improved. High-speed positioning of a machine can thus be achieved by improving the command response characteristics.
For the mechanical system which is a rigid body, high-speed positioning can be achieved by performing model following control in which the motor mechanical model is a rigid body. However, actual mechanical systems have portions with a lower rigidity which cause vibration. In a machine such as a chip mounter, a motor is fixed on a machine base, and a table is driven through a ball screw, as shown in FIG. 3. The machine base is supported by leveling bolts, for example. If the motor is driven to drive the table at a high speed, the machine base sways because of the rigidity of the leveling bolts, causing vibration of the machine base.
In order to suppress such vibration of the machine base, a prefilter may be provided at an input portion of a position command. FIG. 4 is a block diagram of a motor control system that suppresses vibration of a machine base using a prefilter. As the prefilter, a notch filter may be used, for example. Vibration of the machine base can be suppressed by setting the notch frequency of the notch filter to the same value as the frequency of vibration of the machine base. FIGS. 5A and 5B show a differential position command and the positional deviation, respectively, wherein a prefilter is not used. FIGS. 6A and 6B show a differential position command and the positional deviation, respectively, wherein a prefilter is used. Comparing FIGS. 5B and 6B, it can be seen that vibration of a machine base can be suppressed by the use of a prefilter, improving the positioning settling characteristics. However, as can be seen from FIG. 6B, a positioning settling time Ts cannot be sufficiently shortened because of a delay caused by the prefilter.
Vibration of a machine base can also be suppressed using model following control disclosed in Japanese Patent Application Publication No. 2002-163006 (JP2002-163006A). FIG. 7 is FIG. 2 of JP2002-163006A. In a motor control system employing the conventional model following control, a torsion angle compensator 25 is provided in the model control system to suppress vibration. JP2002-163006A, however, does not disclose a specific mathematic model 17 for an apparatus 10 including an electric motor and a machine, although appropriate modeling is essential to suppress vibration of a mechanical system. In the technique disclosed in JP2002-163006A, feedback amounts for the torsion angle and the torsion angle speed, which are used as compensation for vibration suppression control, are adaptively controlled using values of the torsion angle and the torsion angle speed. The parameter for a pseudo speed controller 15 is also adaptively controlled in accordance with a simulated speed deviation. However, JP2002-163006A does not disclose how the parameters for a pseudo-adaptive torsion angle compensator 25 and the pseudo-adaptive speed controller 15 is determined. In order to suppress vibration of a machine base, it is necessary to cause both the gains for the pseudo-adaptive speed controller 15 and the pseudo-adaptive torsion angle compensator 25 to converge to such values that both stabilize the control system and cause no vibration. However, the pseudo-adaptive torsion angle compensator 25 and the pseudo-adaptive speed controller 15 are designed to independently operate to reduce their respective deviations. As a result, even if vibration is suppressed by feeding back the torsion angle and the torsion angle speed, for example, the vibration suppression effect may be impaired if the parameter for the pseudo-adaptive speed controller 15 is changed to restrict the simulated speed deviation. It is thus questionable whether or not vibration of a machine base is really suppressed by the technique disclosed in JP2002-163006A, since it does not clarify a machine model. In addition, because control law between the pseudo-adaptive speed controller 15 and feedbacks of the torsion angle and the torsion angle speed is not disclosed, the pseudo-adaptive torsion angle compensator 25 and the pseudo-adaptive speed controller 15 may fail to converge to suppress vibration.
In contrast, Japanese Patent Application Publication No. 2004-21413 (JP2004-21413A) discloses a conventional technique clarifying a machine model. The technique disclosed in JP2004-21413A uses a machine base model as shown in FIG. 8. In the conventional technique, position information on a machine base and model machine base position information are fed back to suppress vibration. Also, position information on a model, the model machine base position information, and a thrust command are added to position information for a feedback control system and the model machine base position information. In the technique disclosed in JP2004-21413A, a thrust command for the model is thus added to a speed command in the feedback control system through a converter. Therefore, a thrust command in the feedback control system may not match the thrust command in the model control system because of the configuration of a converter or a delay caused by a converter, even if there is no error between the model control system and the feedback control system. Consequently, sufficient model following control characteristics may not be obtained. Moreover, JP2004-21413A does not clarify how the parameters are to be set in order to suppress vibration. Therefore, it may be not be possible to both suppress vibration and achieve high-speed positioning.
The model control system of FIG. 8 is represented symbolically as shown in FIG. 9 wherein a damping term Cb and a viscous resistance Ct are omitted. The parameters are calculated according to the modern control theory as indicated below.
The state equations of the model control system are represented as follows:
                              [                                                                                          x                    .                                    T                                                                                                                          x                    ¨                                    T                                                                                                                          x                    .                                    B                                                                                                                          x                    ¨                                    B                                                              ]                =                                                         [                                                                    0                                                        1                                                        0                                                        0                                                                                                              -                                                                                                    K                            P                                                    ⁢                                                      K                            V                                                                          J                                                                                                                        -                                                                        K                          V                                                J                                                                                                                                                -                                                  1                          J                                                                    ⁢                                              (                                                                                                                                                                                                                  K                                    P                                                                    ⁢                                                                      K                                    V                                                                                                  +                                                                                                                                                                                                        K                                PB                                                                                                                                    )                                                                                                                                                -                                                  1                          J                                                                    ⁢                                              (                                                                              K                            V                                                    +                                                      K                            VB                                                                          )                                                                                                                                  0                                                        0                                                        0                                                        1                                                                                                                                                                K                          P                                                ⁢                                                  K                          V                                                                                            J                        B                                                                                                                                                K                        V                                                                    J                        B                                                                                                                                                1                                                  J                          B                                                                    ⁢                                              (                                                                                                                                                                                                                  K                                    P                                                                    ⁢                                                                      K                                    V                                                                                                  +                                                                                                                                                                                                                                          K                                  PB                                                                -                                                                  K                                  B                                                                                                                                                                    )                                                                                                                                                1                                                  J                          B                                                                    ⁢                                              (                                                                              K                            V                                                    +                                                      K                            VB                                                                          )                                                                                                        ⁢                                                          ]                                             ⁢                                                                                   [                                                                                                              x                          T                                                                                                                                                                                          x                            .                                                    T                                                                                                                                                              x                          B                                                                                                                                                                                          x                            .                                                    B                                                                                                      ]                                +                                                      [                                                                                            0                                                                                                                                                                                                                K                                P                                                            ⁢                                                              K                                V                                                                                      J                                                                                                                                                0                                                                                                                                                  -                                                                                                                            K                                  P                                                                ⁢                                                                  K                                  V                                                                                                                            J                                B                                                                                                                                                                          ]                                    ⁢                                      P                    *                                    ⁢                  y                                            =                                                          ⁢                                                                                ⁢                                                                              ⁢                                                [                                      1                    ⁢                                                                                  ⁢                    0                    ⁢                                                                                  ⁢                    1                    ⁢                                                                                  ⁢                    0                                    ]                                ⁡                                  [                                                                                                              x                          T                                                                                                                                                                                          x                            .                                                    T                                                                                                                                                              x                          B                                                                                                                                                                                          x                            .                                                    B                                                                                                      ]                                                                                        [                  Expression          ⁢                                          ⁢          1                ]            
According to the modern control theory, the control system is stabilized if all the poles have a negative real root. Therefore, the parameters are calculated such that the characteristic equations have a quadruple root as follows:
                              K          =                                    -              4                        ⁢                                                  ⁢                          K              P                                      ⁢                                  ⁢                              K            V                    =                                    4              4                        ⁢                          K              P              3                        ⁢                          J              ·                                                J                  B                                /                                  K                  B                                                                    ⁢                                  ⁢                              K            VB                    =                                                    -                16                            ⁢                                                          ⁢                              K                P                            ⁢                              J                B                                      +                                          (                                                                            J                      B                                        J                                    -                  1                                )                            ⁢                              K                V                                                    ⁢                                  ⁢                              K            PB                    =                                                    -                96                            ⁢                                                          ⁢                              J                B                            ⁢                              K                P                2                                      +                                          (                                                                            J                      B                                        J                                    -                  1                                )                            ⁢                              K                P                            ⁢                              K                V                                      +                          K              B                                                          [                  Expression          ⁢                                          ⁢          2                ]            
Thus, once a position loop gain KP is determined, a root K is determined, which then determines a speed loop gain KV, a machine base speed feedback gain KVB, and a machine base position feedback gain KPB. For example, when KP=125 rad/S in a mechanical system in which JM=8.47×10−4 (Kg·m2), JL=JM×3.66, J=JM+JL, JB=JM×70.5, and KB=1595 (Nm/rad), then KV=3030 Hz, KVB=942 (Nm/(rad/s)), and KPB=4.45×104 (Nm/rad). Thus, when the parameters are calculated so as to both suppress vibration and allow high-speed positioning with the configuration disclosed in JP2004-21413A, the gain for the model speed controller may be a value that is too large to be normally set. In vibration suppression control using model following control, vibration is suppressed by setting a parameter for the model system and a parameter for the feedback control system to the same value. It is therefore necessary to also set the gain for the speed controller in the feedback control system to a large value. However, the gain for the speed controller in the feedback control system is restricted in terms of an upper limit by the rigidity of the mechanical system etc., and thus cannot be set to a significantly large value. Thus, even if the state feedback theory is applied to the model control system of FIG. 8 to calculate such parameter values that are expected to suppress vibration and achieve high-speed positioning, vibration may still be caused, making it difficult to control the motor using the calculation results.
As another method for suppressing vibration of a machine base, it is conceivable to provide a sensor that detects vibration of the machine base. However, the sensor may reduce the reliability of the method in case of a failure, and may also increase the cost.